hidden mass extinction
Biosphere Daily Earth Science Repost

A hidden mass extinction event

No evidence exists today of their presence, some 3.5 to 4.5 billion years ago. Likewise, their demise is obscured in the geological record. But the case for chemosynthetic life forms being first comers to the biosphere is strong. If they existed, then their disappearance into the void of time constitutes the first great mass extinction on the planet.

The period in question was during the Hadean Eon and subsequent Archean Eon—a vast stretch of time from 4.5 billion to 2.5 billion years ago. But the timeframe of most interest is the first billion years of the earth’s existence, and it only includes the earliest period of the Archean.

Scientists, conducting research in Greenland during early 2016, detected early life remnants in rocks as old as 3.7 billion years. The structures they identified are called stromatolites, which appear in the rocks as rumpled layering. The layers are fossil traces of algal mats from an ancient seafloor. Blue-green algae, or cyanobacteria, were responsible for creating the sediment layers, which became buried beneath thick piles of other sediments before eventually turning into rock. Importantly, the algae creating these rock layers were complex cellular organisms with the ability to produce energy from sunlight via photosynthesis.

Model T to Lamborghini

No direct evidence exists of an early Hadean/Archean world populated by chemosynthetic bacteria. But a direct evolutionary jump from organic molecules to DNA-laden cyanobacteria seems unlikely. Such an evolutionary leap would be like Henry Ford setting up his factory and then having a modern-day Lamborghini as the first car off the assembly line. However, you don’t go from Model T technology to a Lamborghini in one step.

 Photosynthesis produces oxygen as a byproduct. But before cyanobacteria, the atmosphere and oceans contained no free oxygen.  How did life start in a world devoid of oxygen and organic molecules? The first organisms to develop a cellular structure needed energy. However, the only raw materials these first bacteria had to work with were inorganic molecules. Chemosynthetic bacteria are the most likely answer to this riddle.

But first, Greenland

Allen Nutman and his research team traveled to Greenland in 2016, searching for evidence of ancient life. Prior to their expedition, the oldest discovered hints of early life came from the Strelley Pool Chert of western Australia—a rock formation containing 3.4 billion-year-old stromatolites.

Nutman and his colleagues traveled to the Isua Greenstone Belt, located in the Nuuk region on the southwest coast of Greenland. The town of Nuuk is situated on the coast, squeezed between the Labrador Sea and an inland ice cap. But between the town and the ice is a stretch of exposed rock called the Isukasia Terrane, where 3.7 billion-year-old rocks lie exposed at the earth’s surface.

A paradise for geologists consists of a landscape of pure rock with no soil or vegetative cover blocking the view. The Isukasia Terrane comes close to being a paradise. With a stroke of good fortune, the Nutman party discovered a stunning rock exposure, freshly revealed from beneath newly melted snow. This rock outcrop contained an impressive set of light, reddish-brown rock layers set against a gray background (see the photo in an excellent article by Carolyn Gramling).

Nutman and others believe these layered features in the Isua rocks are stromatolites. However, some experts in the field of early life have posed valid questions about the find and are awaiting more verification before casting a final opinion. Science is an arena where intelligent debate freely occurs—probably contributing to its absence in much government policymaking today. The investigations are still in progress, and the final jury is out, but detectable, photosynthesizing, oxygen-producing life may have inhabited the early earth’s shallow seas 3.7 billion years ago.

Chemosynthetic life

Chemosynthesis refers to the process of producing food using chemicals as opposed to sunlight. Chemosynthetic bacteria use this process, deriving their energy from converting inorganic molecules into organic substances. They don’t need sunlight, and they don’t require existing organic material. Their energy derives from the chemical oxidation of available inorganic molecules, and they feed on substances such as ammonia, molecular hydrogen, sulfur, hydrogen sulfide, and ferrous iron. 

Fortunately for these ancient creatures, the chemicals they needed were abundant in the early Archean oceans. Their food source was pumped into the oceans via volcanic and hydrothermal vents. So the dark, lightless, habitats around volcanic vents in the deep oceans provided an oasis of life for these tough little chemosynthetic bacteria. 

Chemosynthetic bacteria thrive today in some of the most inhospitable environments on the earth. They call hydrothermal vents at the bottom of the ocean home. The bone-crushing pressures and hellish temperatures around these vents make them happy. They also thrive deep in the polar ice, hidden in remote caves, and even they even flourish petroleum deposits. Chemosynthetic organisms come with a set of skills, making them ideal for the harsh conditions on the early earth.

These soft, single-celled, chemosynthetic organisms probably lived, died, and evolved for a billion years before the first photosynthesizing blue-green algae started producing free oxygen. 

Free oxygen

When we talk about breathing oxygen, we mean free oxygen as opposed to oxygen bonded to other elements like carbon or nitrogen. Free oxygen is what most animal life needs to survive in our world today.

Aerobic and anaerobic describe two fundamentally different environments. Aerobic organisms thrive in the presence of oxygen, but environments with no free oxygen are only hospitable to anaerobic microorganisms. Also, many anaerobic bacteria die in the presence of oxygen. Free oxygen is toxic to them. The oxygen issue is essential when considering early life. Because the early atmosphere and oceans were anoxic, the chemosynthetic organisms probably had no defense against oxygen toxicity. 

Chemosynthetic organisms stayed at the top of the evolutionary pyramid until cyanobacteria (blue-green algae) learned the trick of photosynthesis some 3.4 to 3.7 billion years ago. The atmosphere at the beginning of the Archean Eon was anoxic, lacking any significant component of free oxygen. But when blue-green algae appeared on the scene, they thrived under the eye of an ancient sun. They used their photosynthesis trick to convert free solar energy into chemical energy.

Death by oxygen

For a billion years, these blue-green algae dominated the biosphere. Ever so slowly, molecule by molecule, the algae took CO2 + H2O + sunlight and produced the sugars needed for life. However, all actions have unintentional consequences, and a byproduct of photosynthesis was the free oxygen (O2) the algae discarded. A fully oxygenated atmosphere would not appear until the Great Oxygenation Event about 2.4 billion years ago. However, this slow leakage of oxygen, first into the oceans and later into the atmosphere, was toxic to much of the existing chemosynthetic life. 

Their soft remains rained onto the ocean floor as they died and disintegrated, leaving no fossilized traces of life’s earliest ecosystem. Perhaps up to a billion years of evolution disappeared in the newly oxygenated biosphere. The Isua Greenstone Belt contains the oldest known sedimentary rocks on the plant. Even if the early chemosynthetic empire left fossilized remains, the rocks don’t exist to track them back to their origin. 

Blue-green algae didn’t suddenly pop into existence, arising from a lifeless void. Something proceeded them, and those creatures were most certainly chemosynthetic lifeforms. But they are gone now, wiped out in a hidden mass extinction event.

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.